Polyurethanes are employed in a wide range of applications, notably in the form of foams. Depending on their composition, polyurethane foams can vary in structure from soft flexible foams used as cushioning materials to rigid foams used in insulating or structural materials.
Polyurethane foams are most often obtained by polymerization between a polyisocyanate and a hydroxyl terminated oligomer (polyol) in the presence of water. The reaction between polyol and isocyanate forms urethane linkages, while the reaction between polyisocyanate and water yields polyurea and gaseous carbon dioxide, the latter causing foaming of the composition.
Instead of water, other blowing agents may be used to perform expansion and foaming of the polymeric matrix. The blowing agent may be either produced in situ as a reaction product of the reactants (like water and isocyanate producing carbon dioxide). These blowing agents are called chemical blowing agents. Or the blowing agent may be a physical blowing agent, i.e. a non-reactive compound contained in the polymeric composition that is able to generate bubbles in the polymeric matrix during its formation, thereby leading to foam.
Polyurethane foams derived from polyisocyanates are associated with environmental issues because isocyanates raw materials, in particular methylene diphenyl 4,4′-diisocyanate (MDI) and toluene diisocyanate (TDI), the most widely used isocyanates in the polyurethane industry, and the corresponding aromatic diamines are classified as CMR (carcinogenic, mutagenic and reprotoxic substances).
Therefore, there is a need to produce polyurethane foams which are not derived from polyisocyanates, i.e. non isocyanate polyurethane foams (NIPU).
Cornille et al. (“A new way of creating cellular polyurethane materials: NIPU foams”, European Polymer Journal, 66 (2015) 129-138) teaches the synthesis of polyurethane foams by reacting a tri- and difunctional mixture of five-membered cyclic carbonates with diamines to yield NIPU foams. The mixture of carbonates comprises a trifunctional carbonate (trimethylolpropane tris carbonate (TMP-Tri-C5)) and a difunctional carbonate (polypropylene oxide bis-carbonate (PPO-Bis-C5)). A poly(methylhydrogenosiloxane) is used as a chemical blowing agent to foam the NIPU by reaction with the diamines.
However, the synthesis described in this article suffers from important shortcomings from an industrial perspective. Indeed, the chemical blowing agent generates the release of hydrogen which is very flammable and thus risky and complicated to handle on an industrial scale. Furthermore, since the blowing agent is incorporated into the backbone of the polymer, there are limitations regarding maximal content of the blowing agent, thus limitations regarding foam properties, density and mechanical properties of the final product. The synthesis described in this article only enables the preparation of high density foams with apparent densities ranging from 194 to 295 kg/m3. In this respect, the article teaches that the use of only tri-functional cyclic carbonate increases the apparent density of foams.
Therefore, there is still a need to develop a process making it possible to prepare NIPU foams that is easy to implement on an industrial scale, without the generation of flammable and explosive gas, and that also enables the preparation not only of high density flexible NIPU foams but also low density flexible NIPU foams.
The inventors of the present invention have succeeded in synthesizing NIPU foams meeting all these needs by reacting a multifunctional amine with multifunctional cyclic carbonates and alkoxylated derivatives thereof and by using a non-reactive blowing agent.
The NIPU foams of the invention have the advantage that they can be prepared in whole or in significant part from renewable domestic agricultural materials and thus qualify as biobased products.